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United States Patent |
5,218,532
|
Mori
|
June 8, 1993
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Method and system for acquiring MR data in MRI
Abstract
A change, e.g., a cardiac arrhythmia or a motion in a subject is monitored
during an acquisition of MR data in an MRI apparatus, and it is determined
whether or not a change occurs in the subject. If a change occurs, after
an acquisition of the MR data, the MR data is reacquired in the encoding
process when the change occurs, and the MR data in the corresponding
encoding process is exchanged for the reacquired MR data. An MR image is
reconstructed in accordance with MR data including the exchanged MR data,
and an MR image which does not include an artifact due to a change in the
subject is displayed.
Inventors:
|
Mori; Akio (Ootawara, JP)
|
Assignee:
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Kabushiki Kaisha Toshiba (Kawasaki, JP)
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Appl. No.:
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489901 |
Filed:
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March 7, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
600/410; 324/309; 600/509 |
Intern'l Class: |
G01R 033/20 |
Field of Search: |
364/413.13,413.02,413.03,413.15,413.2
324/307,309,313
128/653 A,696,702,708
|
References Cited
U.S. Patent Documents
4719424 | Jan., 1988 | Jimbo et al. | 324/309.
|
4751462 | Jun., 1988 | Glover et al. | 324/309.
|
4761613 | Aug., 1988 | Hinks | 324/309.
|
4830012 | May., 1989 | Riederer | 128/653.
|
4855910 | Aug., 1989 | Bohning | 364/413.
|
4994965 | Feb., 1991 | Crawford et al. | 364/413.
|
Primary Examiner: Smith; Jerry
Assistant Examiner: Gordon; Paul
Attorney, Agent or Firm: Foley & Lardner
Claims
What is claimed is:
1. A method for acquiring MR data in MRI, said method comprising the steps
of:
acquiring first MR (magnetic resonance) data of a subject by performing a
desired sequence having a plurality of encoding processes;
monitoring a state of said subject during acquistion of said first MR data;
storing data associated with an encoding process of said encoding processes
performed when a change in said state of said subject occurs;
repeating said acquiring step, said monitoring step and said storing step a
plurality of times;
acquiring second MR data of said subject by performing said encoding
process after acquistion of said first MR data is completed when said
change in said state of said subject occurs;
changing at least one data item of said first MR data in said encoding
process into zero data;
averaging said first MR data other than said at least one data item in said
encoding process and said zero data to obtain averaged first MR data;
dividing said second MR data by a predetermined value to obtain a quotient
of MR data;
adding said quotient of MR data to said averaged first MR data in said
encoding process to obtain first summation MR data; and
reconstructing an MR image from said first summation MR data.
2. The method according to claim 1, wherein said monitoring step includes a
step of detecting electrocardiogram signals from said subject.
3. The method according to claim 1, wherein said monitoring step includes a
step of detecting motion of said subject.
4. A method for acquiring MR data in MRI, said system comprising:
means for acquiring first MR data of a subject by performing a desired
sequence having a plurality of encoding processes;
means for monitoring a state of said subject during acquistion of said
first MR data;
means for storing data associated with an encoding process of said encoding
processes performed when a change in said state of said subject occurs;
means for acquiring second MR data of said subject by performing said
encoding process after acquistion of said first MR data is completed when
said change in said state of said subject occurs;
means for changing at least one data item of said first MR data in said
encoding process into zero data;
means for averaging said first MR data other than said at least one data
item in said encoding process and said zero data to obtain averaged first
MR data;
means for dividing said second MR data by a predetermined value to obtain a
quotient of MR data;
mean for adding said quotient of MR data to said averaged first MR data in
said encoding process to obtain first summation MR data; and
means for reconstructing an MR image from said first summation MR data.
5. The system according to claim 4, wherein said monitoring means includes
means for detecting electrocardiogram signals from said subject.
6. The system according to claim 4, wherein said monitoring means includes
means for detecting motion of said subject.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and system for acquiring magnetic
resonance (MR) data in magnetic resonance (MR).
2. Description of the Related Art
An atomic nucleus, having a spin and a magnetic moment which are not zero,
in a static field, resonance-absorbs and radiates only an electromagnetic
wave with a specific frequency by a magnetic resonance phenomenon. This
atomic nucleus resonates at an angular frequency .omega.0
(.omega.0=2.pi..nu.0; .nu.0 is the Larmor frequency) defined as follows:
.omega.0 =.nu.H0
where .nu. is the gyromagnetic ratio which is inherent for each kind of an
atomic nucleus, and H0 is the intensity of a static field.
In a magnetic resonance imaging (MRI) apparatus for diagnosing living
bodies utilizing the magnetic resonance phenomenon, an MR signal induced
after resonance absorption is detected and processed to acquire, without
invasion, diagnosis data such as a slice image (MR image) of subject in
accordance with, e.g., a density of atomic nuclei, longitudinal and
transverse relaxation times periods, flow, and chemical shift.
Diagnosis data can be obtained on the basis of an MR signal generated by
exciting the entire subject placed in a static field. In the conventional
MRI apparatus, however, diagnosis data is obtained on the basis of an MR
signal generated by exciting a specific portion of the subject in
accordance with various limitations of the apparatus used and a specific
clinical demand for an MR image.
In such an MRI apparatus, in order to reduce an artifact of an MR image due
to motion and blood flow of a subject, beat-sync scanning is performed. In
beat-sync scanning, for example, an R-wave of an electrocardiogram signal
detected from the subject, on whom an electrocardiogram lead electrode is
mounted, is used as a scanning sync signal, and MR data is acquired in
accordance with the scanning sync signal.
Scanning for acquiring MR data in accordance with a beat sync signal to
reconstruct one MR image will be described hereinafter with reference to
FIG. 1.
As shown in FIG. 1, after a predetermined period of time (application
timing period) Td elapses from generation of an R-wave of an
electrocardiogram signal of a subject P, a trigger signal is generated to
apply an RF (radio frequency) pulse by a spin echo method. The RF pulse is
applied to a slice portion SL of the subject P in response to the trigger
signal. An MR signal generated from the slice portion SL upon application
of the RF pulse is acquired as MR data in a first encoding process. When
RF pulses are sequentially applied to the slice portion, MR data of
second, third, . . . encoding processes are acquired.
For example, when an MR image having a 256.times.256 matrix is
reconstructed, application of an RF pulse is repeated 256 times at a
timing of generation of each trigger signal. Therefore, MR data associated
with 256 encoding processes are acquired.
Note that, when intervals between adjacent R-waves are constant, a
repetition time Tr of the RF pulse is also constant. However, when the
intervals between R-waves are changed due to, e.g., arrhythmia in the
fourth beat, the repetition time Tr of the RF pulse is changed into a
repetition time Tr'.
When the repetition time Tr of the RF pulse is changed, the amplitude and
phase of the MR signal are changed. MR data is changed in an encoding
direction on an MR image in accordance with a change in amplitude and
phase of the MR signal. Therefore, an artifact occurs in the encoding
direction.
An averaging process is not taken into consideration in the above-mentioned
MR data acquisition. However, when MR data is acquired in practice, an
averaging process is performed in order to improve the S/N ratio. More
specifically, a plurality of MR data acquired in a single encoding process
are averaged. Therefore, the amplitude of a normal MR signal is always
constant.
On the other hand, since the amplitude of a noise signal is irregular, the
amplitude of the added noise signal is much less than that of the normal
MR signal. Therefore, when the noise signal is averaged together with
normal signal components, only a substantially normal MR signal can be
acquired, thereby improving the S/N ratio.
However, during acquisition of MR data, if cardiac arrhythmia occurs in a
subject or the subject moves, the amplitudes of MR signals obtained by the
single encoding process are changed. Therefore, when the averaging process
is performed, an S/N ratio is reduced.
In addition, when a change in the subject, e.g., cardiac arrhythmia or
motion, is not monitored, an operator cannot determine whether or not the
acquired MR image includes an artifact caused by a change in the subject.
Furthermore, if cardiac arrhythmia occurs in the subject or the subject
moves, MR data must be reacquired. Therefore, it takes a long time period
to acquire MR data.
Thus, an MRI apparatus is desirable which can show a change result of a
subject to an operator if cardiac arrhythmia occurs in the subject or the
subject moves during an acquisition of MR data, and which can reacquire MR
data in the same encoding process when the subject is changed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and system for
acquiring MR data in MRI.
According to one aspect of the present invention, there is provided a
method for acquiring MR data in MRI, the method comprising the steps of:
acquiring first MR data of a subject in accordance with a desired sequence
having a plurality of encoding processes;
monitoring a state in the subject during an acquisition of the first MR
data;
obtaining the encoding process when a change in the subject is detected;
acquiring second MR data in accordance with the obtained encoding process
after acquisition of the first MR data is completed when the change in the
subject is detected;
processing the first MR data in the obtained encoding process in accordance
with the acquired second MR data; and
reconstructing an MR image from the processed first MR data.
According to another aspect of the present invention, there is provided a
system for acquiring MR data in MRI, the system comprising:
means for acquiring first MR data of a subject in accordance with a desired
sequence having a plurality of encoding processes;
means for monitoring a state in the subject during an acquisition of the
first MR data;
means for obtaining the encoding process when a change in the subject is
detected;
means for acquiring second MR data in accordance with the obtained encoding
process after the acquisition of the first MR data is completed when the
change in the subject is detected;
means for processing the first MR data in the obtained encoding process in
accordance with the acquired second MR data; and
means for reconstructing an MR image from the processed first MR data.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate a presently preferred embodiment of the
invention and, together with the general description given above and the
detailed description of the preferred embodiment given below, serve to
explain the principles of the invention.
FIG. 1 is a view showing a slice portion of a subject, and a timing chart
showing application timings of electrocardiogram signals and RF pulses;
FIG. 2 is a block diagram showing an arrangement of a system according to
an embodiment of the present invention;
FIG. 3 is a flow chart for explaining an operation of a process and control
unit in the system according to the present invention;
FIG. 4 is a flow chart of the process routine R1;
FIG. 5 is a display example on a display unit after the process routine R1
is executed;
FIG. 6 is a flow chart of the process routine R2;
FIG. 7 is a view for explaining MR data acquisition by the process routine
R2;
FIG. 8 is a flow chart of the process routine R3; and
FIG. 9 is a view for explaining MR data acquisition by the process routine
R3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be described hereinafter with
reference to the accompanying drawings.
As shown in FIG. 2, a system according to the present invention includes a
magnetic field generating coil unit 1, a static field source 2, a gradient
field source 3, a transmitting and receiving unit 4, a scan controller 5,
a process and control unit 6, a memory 7, an image reconstructing unit 8,
a display unit 9, an electrocardiogram signal detector 10, and a motion
detector 11.
The magnetic field generating coil unit 1 includes a static field coil 1a,
gradient field coils lb, and an RF coil 1c. A subject P is positioned in
the coil unit 1. Note that either a transmitting and receiving coil or a
transmitting coil and a receiving coil are used as the RF coil 1c.
The static field source 2 supplies power to the static field coil 1a for
generating a static field.
The gradient field source 3 supplies power to the gradient field coils 1b
for generating gradient fields in x, y and z directions.
The transmitting and receiving unit 4 transmits an RF pulse to the RF coil
1c so as to apply an RF pulse to the subject P and receives an MR signal
detected by the RF coil 1c.
The scan controller 5 controls the gradient field source 3, the
transmitting and receiving unit 4, the electrocardiogram signal detector
10, and the motion detector 11 in accordance with a predetermined control
sequence.
The process and control unit 6 controls the scan controller 5, the memory
7, the image reconstructing unit 8, and the display unit 9.
The memory 7 stores MR signals acquired in each encoding process as MR
data. Note that the memory 7 also stores data associated with scanning
conditions e.g., a scan portion of the subject P.
The image reconstructing unit 8 reconstructs an MR image by performing,
e.g., Fourier transform with respect to the MR data stored in the memory
7.
The display unit 9 displays the MR image reconstructed by the image
reconstructing unit 8.
The electrocardiogram signal detector 10 detects an electrocardiogram
signal from the subject P on whom an electrocardiogram lead electrode 10a
is mounted, and generates a beat sync signal shown in FIG. 1 as a trigger
signal on the basis of the electrocardiogram signal. The trigger signal is
input to the scan controller 5 to generate an RF pulse. In addition, the
electrocardiogram signal detector 10 detects the presence/absence of
cardiac arrhythmia on the basis of the detected electrocardiogram signal
and outputs data representing the detection result to the scan controller
5.
The motion detector 11 detects the presence/absence of a motion of the
subject P and outputs data representing the detection result to the scan
controller 5. For example, optical transmitters 15a and 15b and optical
receivers 16a and 16b are arranged at predetermined positions around the
subject P. The motion detector 11 detects whether or not light
respectively emitted from the transmitters 15a and 15b toward the
receivers 16a and 16b are interrupted by the subject P. Data representing
the determination result is input to the scan controller 5.
An operation of this system will be described hereinafter.
In this system, acquisition of MR data, detection of a change in the
subject, and reconstruction of an image are performed following a flow
chart shown in FIG. 3.
In step S1, the process routine R1, R2, or R3 is set. The process routines
R1, R2, and R3 are executed if cardiac arrhythmia occurs in the subject or
the subject moves during an acquisition of an MR signal from a
predetermined portion of the subject in synchronism with heart beats of
the subject.
In step S2, an MR data acquiring process is performed under the control of
the scan controller 5. The acquired MR data is stored in the memory 7. In
addition, a change in the subject during an acquisition of MR data is
monitored by the electrocardiogram signal detector 10 and the motion
detector 11.
In step S3, it is determined whether or not a change in the subject is
detected during MR data acquisition.
If a change in the subject is not detected during MR data acquisition in
step S3, a reconstruction process is performed by 2 dimensional Fourier
transform in the image reconstructing unit 8 in step S8. In step S9, the
reconstruction image is displayed by the display unit 9.
When a change in the subject is detected during MR data acquisition in step
S3, data associated with the encoding process performed when the change
occurs is stored in the memory 7.
In step S4, it is determined which process routine is set in step S1.
If the process routine R1 is set, the process routine R1 is executed in
step S5 in accordance with a flow chart shown in FIG. 4.
In step A1, data associated with the encoding process is read out from the
memory 7, and the readout data is input to the display unit 9.
In step A2, a warning message is generated in accordance with the detection
result, and the obtained message is input to the display unit 9.
After the process routine R1 is completed, the image reconstruction process
is performed in step S8. As shown in FIG. 5, in step S9, the readout data
associated with the encoding process and a warning message 21 are
displayed on the display nit 9 together with a reconstructed image 20.
If it is determined in step S4 that the process routine R2 is set, the
process routine R2 is executed in step S6 in accordance with a flow chart
shown in FIG. 6.
In step B1, data associated with the encoding process stored in the memory
7 is read out, and MR data in this encoding process is reacquired.
As shown in FIG. 7, in step B2, the reacquired MR data is exchanged for the
MR data previously acquired in the corresponding encoding process.
After the process routine R2 is completed, the image reconstruction process
is performed (step S8), and the reconstructed image is displayed (step
S9).
If it is determined in step S4 that the process routine R3 is set, the
process routine R3 is executed in step S7 in accordance with a flow chart
shown in FIG. 8. Note that FIG. 9 shows a case wherein the averaging
process is performed four times for MR data acquistion in each encoding
process.
In step C1, data associated with the encoding process stored in the memory
7 is read out, and the MR data acquired in this encoding process is
exchanged for zero.
In step C2, MR data in this encoding process is reacquired.
In step C3, the reacquired MR data is divided by a predetermined value. In
addition, the divided MR data is added to the MR data which is previously
acquired in the corresponding encoding process. Note that the
predetermined value is set in advance in accordance with the data
associated with the encoding process stored in the memory 7.
After the process routine R3 is completed, an image reconstruction process
is performed (step S8), and the reconstructed image is displayed (step
S9).
Thus, if cardiac arrhythmia occurs in the subject or the subject moves, an
operator can know than the displayed reconstructed image includes an
artifact. In addition, a reconstructed image which does not include an
artifact due to a change in the subject can be displayed.
Although execution of one process routine of the process routines R1, R2,
and R3 has been described in this embodiment, the process routine R1 can
be combined with the process routine R2, or the routine R1 can be combined
with the process routine R3.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the invention in its broader aspects is not limited
to the specific details, representative devices, and illustrated examples
shown and described. Accordingly, various modifications may be made
without departing from the spirit or scope of the general inventive
concept as defined by the appended claims and their equivalents.
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